Journal Impact Factor (JIF) is the most well known journal-level metric. In its infancy it was used to help make decisions on which journals should be included in a library collection. The calculation for JIF is citations garnered in the last two years divided by the number of articles in the last two years for a specific journal.
Due to this simplicity of this calculation, please be mindful that this quantitative measurement does not tell the whole story of a Journal's prestige and critical appraisals should not be based on this number alone.